1. Field of the Invention
This invention relates in general to semiconductor devices, and in particular to an improved method for use in manufacturing such devices.
2. Discussion Relative to the Prior Art
The manufacture of semiconductor devices such as silicon integrated circuits and charge coupled devices requires that impurities be kept from residing on the surface of such devices. Impurities such as copper, iron and gold are especially troublesome since they form undesirable charge traps at device surfaces, and can diffuse into the silicon bulk to form hot spots, or noise sources, which adversely affect device performance.
Aside from ab initio attempts to minimize impurity contamination, present practice is to getter impurity atoms at various stages of device manufacture. Conventionally, devices are formed from a silicon wafer which is photolithographically processed through successive steps, many of which involve
1. THERMAL DIFFUSION OF DOPANTS INTO THE SILICON, AND
2. THE THERMAL GROWTH OF AN OXIDE OVER REGION(S) OF THE SILICON.
Before each thermal step (which takes place in a gaseous environment within a high-temperature oven), it is customary to clean the silicon wafer. Typically, this is done by immersing the wafer in a dilute solution of ammonium hydroxide and hydrogen peroxide for 20 minutes at 80.degree. C.; rinsing the wafer with distilled water; immersing the wafer in a solution of hydrogen peroxide and hydrochloric acid for 20 minutes at 80.degree. C.; and again rinsing the wafer with distilled water. Such a technique, while working to clean away trace amounts of, impurities leaves something to be desired, primarily because it requires handling and processing by wet solutions; and because it takes a fair amount of time to do. (A wafer, before processing, usually has impurity levels lower than after such processing, despite the use of cleaning procedures. For example, an unprocessed wafer may have a gold impurity level of 10.sup.11 atoms/cc of silicon; after processing -- with cleaning -- the gold impurity level might rise, say, to 6 .times. 10.sup.12 atoms/cc of silicon.)